1. Techincal Field of the Invention
The present invention relates to a method for manufacturing a ceramic-based composite material, in which the conversion ratio of an organic silicon polymer to SiC is increased.
2. Prior Art
Ceramics are highly resistant to heat, but are brittle. Therefore, ceramic-based composite materials (Ceramic Matrix Ceramics: CMC) reinforced by ceramic fibers have been developed. When manufacturing CMC, a matrix formation process whereby a matrix is created between gaps of formed fiber fabric is configured with, for instance, a chemical vapor infiltration (CVI) process and a polymer impregnation and pyrolysis process (PIP).
In the CVI process, the material is processed with the vapor of a chemical substance, and a dense ceramics matrix is created on the surface of the fiber fabric. The PIP is a process whereby an organic silicon polymer is dissolved in a solvent such as xylene, and is impregnated into a fabric infiltrated with the chemical vapor and pyrolyzed in an inert gas, the fabric is fired at a high temperature to convert the organic silicon polymer into SiC.
A ceramic-based composite material that requires airtightness, used in, for example, a turbine blade, thrust chamber, or nozzle, must be filled completely with the matrix in the gaps between fibers of the CMC fabric by repeating the aforementioned PIP process.
However, according to such a conventional method for manufacturing ceramic-based composite materials, the conversion ratio from the organic silicon polymer into SiC during high-temperature is as low as 30 to 60%, therefore the PIP process must be repeated alternately by about 20 times, for instance. As a consequence, a fairly long time is required for repeated PIP processes, resulting in the low production efficiency of the ceramic-based composite material, as an undesirable practical problem.
The present invention was achieved to solve this problem. More explicitly, an object of the present invention is to offer a method for manufacturing a ceramic-based composite material by increasing the conversion ratio of an organic silicon polymer to SiC, increasing the infiltration efficiency of SiC in a PIP process, so that an airtight ceramic-based composite material can be manufactured efficiently within a short time.
Organic silicon polymers conventionally processed by the above-mentioned PIP process include, for example, polycarbosilane (PCS) and polymethylsilane (PMS). Polycarbosilane (PCS) is a polymer polymerized from its precursor, dimethylchlorosilane, however, because surplus H atoms and C molecules contained in PCS molecules are decomposed as hydrogen and methane in the initial period of the inert gas firing process, conventional conversion ratios to SiC are approximately 60%. In addition, polymethylsilane (PMS) is a polymer polymerized from its precursor, monomethylchlorosilane, but because PMS is a low-molecular-weight oligosilane, the conversion ratio to SiC is still lower, e.g., about 40% at maximum. In addition, the SiC produced contains much Si.
According to the present invention, polycarbosilane (PCS) and polymethylsilane (PMS) are mixed beforehand to protect a conventional organic silicon polymer from decomposition during the initial period of the inert gas firing process to make PCS and PMS crosslinked together, and the conversion ratio to SiC is increased.
In detail, according to the present invention, the method for manufacturing a ceramic-based composite material is configured with a fiber fabric formation process whereby the fiber fabric is formed with inorganic fibers, a fiber surface treatment process that forms a coating layer on the surface of the aforementioned fabric, and a matrix formation process in which a matrix is produced between the fibers; the above-mentioned matrix formation process is composed of a crosslink process in which a mixed organic polymer containing at least polycarbosilane (PCS) and polymethylsilane (PMS) is maintained for a predetermined time at an intermediate temperature which is lower than the firing temperature of the polymer, and PCS and PMS are crosslinked together, a PIP process in which the organic polymer is impregnated into the material, and an inert firing process whereby the material is fired at a high temperature in an environment with an inert gas, after the previous crosslink process.
According to this method, the polycarbosilane (PCS) and polymethylsilane (PMS), mixed in a polymer mixing process, are crosslinked in molecules together in the crosslink process, into a polymer with a Sixe2x80x94C rich skeleton. The subsequent loss during high-temperature firing in the inert gas is reduced, so the conversion ratio to SiC can be increased.
According to a preferred embodiment of the present invention, the mix of PCS and PMS is maintained at about from 573K to 723K for a predetermined time in the aforementioned crosslink process. An experiment using this method revealed that a conversion ratio to SiC of about 90% could be achieved.
Other objects and advantageous features of the present invention will be understood referring to the following description with reference to the attached drawings.